A group of frogs and humans gathered in a Yosemite Valley meadow on a blue-skied spring day before the backdrop of thundering Yosemite Falls.
The frogs likely didn’t know this was a huge historic afternoon, as Yosemite National Park spokesman Scott Gediman called it, but they could sense pond water nearby while awaiting their release from plastic Ziploc containers, noted Rob Grasso, a park aquatic ecologist.
It had been a long journey from San Francisco, where the federally threatened California red-legged frogs were reared.
On Friday, as they were released into Yosemite Valley ponds, the frogs joined a growing population of red-leggeds in Yosemite following a 50-year absence in the park. An amphibian fungus and population of non-native predatory bullfrogs were part of wiping them out in this High Sierra wonderland. Now they’re back – and thriving. This is the first year of documented breeding by the frogs in Yosemite since adult red-leggeds started to be reintroduced here in 2017.
Her organization, a philanthropic partner of Yosemite, has contributed $570,000 from donors over the past three years to protect aquatic species in the park. The frog programs received $130,000 from the conservancy in 2019.
Around 200 adult frogs were released in several Yosemite Valley sites last week, including Cook’s, El Cap and Leidig meadows. Another 500 have been released since 2017. All are microchipped. Thousands of frog eggs and tadpoles have also been released.
Why do California red-legged frogs matter?
The California red-legged frog was made famous by author Mark Twain, who took notice of this striking amphibian, painted in colors of red, gold, black and green, in his short story, “The Celebrated Jumping Frog of Calaveras County.” But beyond their fine frog looks and important role in any ecosystem, frogs help people, too.
“They are a sentinel, both of land, air and water quality,” Grasso said. “So if water quality is poor, frogs tend not to do well. And since they have a bilateral – a water and land life stage – you can actually look for indicators on land that might not be healthy, and that can be a lot of indicators related to habitat. So I used to often jokingly say to people, ‘It’s the frogs now, but you could be next.’”
Frogs are more sensitive to any contaminants in water, which they absorb and can lead to death. Grasso compared frogs to canaries that coal miners used to bring underground to watch for deadly levels of contaminants.
“Frogs are that same sentinel,” he said. “If they’re not doing well and we don’t really understand why, it’s kind of a red flag that we should investigate that.”
Frogs with microchips, radio transmitters
All of the adult red-legged frogs released in Yosemite have been implanted with an 8-millimeter microchip, similar to what’s done for many pets. The microchips contain data about the frogs that researchers can scan if found in the wild.
In June, another 275 frogs will be released in Yosemite. Seventy-five of them will be fitted with battery-powered radio transmitters for the first time, secured with small beaded chains like a belt or necklace. The transmitters allow researchers to track the frogs from a quarter mile away and will be removed in December, Grasso said.
These are helpful tools for researchers since the red-leggeds are largely nocturnal and prone to hiding. They’re also hard to hear since vocalization occurs underwater, and the season for mating calls usually only lasts from March to April. Grasso describes the sound like someone rubbing a balloon.
Grown from eggs, tadpoles in San Francisco
The San Francisco Zoological Society and Yosemite National Park Conservation and Recovery Facility, opened in San Francisco in 2016, reared the frogs. Jessie Bushell, director of conservation for San Francisco Zoo and Gardens, said staff had minimal interactions with them to ensure the frogs are still afraid of people and predators in the wild.
The frog eggs were gathered from private property in Garden Valley northeast of Sacramento. Grasso started looking there after a red-legged frog was found nearby in Eldorado National Forest by coworkers before he started working in Yosemite.
Only a small percentage of eggs were taken, and shouldn’t impact the Garden Valley population, researchers said.
It’s hard to know exactly how many have survived in Yosemite, or how the deadly amphibian fungus– believed to have been brought to the area from Africa or Korea – will continue to impact them. But Grasso takes heart seeing they’re healthy enough to breed. Red-leggeds without microchips were also found in Yosemite Valley’s Mirror Lake, suggesting tadpoles released there in 2016 survived.
Grasso has seen 20 red-legged egg masses in Yosemite Valley this year, which could mean around 50,000 tadpoles – although only about 1% to 3% of tadpoles typically survive in the wild. Many more egg masses are believed to exist.
What happened to the American bullfrogs?
One of the red-leggeds’ greatest foes, the American bullfrog, was declared eradicated in Yosemite Valley by 2014.
About 1,000 adult American bullfrogs were killed in Yosemite over a 10-year period, Grasso said.
Yosemite got serious with funding this eradication effort in the 2000s, Grasso said, after a chief of wildlife didn’t like the unnatural soundscape created by the bullfrogs, believed to have been introduced in Yosemite Valley in the early 1950s.
Water samples still show American bullfrog DNA in Yosemite but park researchers haven’t seen them for four years, Grasso said. Any bullfrogs that may still exist are no longer considered a serious threat.
The red-leggeds’ decline throughout California also was caused by wetland habitats being damaged and depleted of water, and the frogs being killed for food in the 19th and 20th centuries. American bullfrogs were then brought in for food – the “nail in the coffin,” Grasso said. Raccoons also have been formidable predators.
Grasso said the bullfrogs in Yosemite were killed using the most humane methods possible. A light is shined in a bullfrog’s eyes, causing it to freeze. A bullfrog is then picked up and its belly rubbed with Orajel, which puts it to sleep and can stop its heart, Grasso said. A sharp object is then used to cut the bullfrog at the base of its skull, he said.
Invasive species management
Why eradicate invasive, non-native species?
“I think there are fair arguments that are transpiring about where you do this type of work and where you don’t,” Grasso said. “Obviously in a national park, you know the land is going to be set aside in perpetuity, that it makes a lot of sense to do it here.”
He said ecologists are now struggling with this question: “How do you value one species over another and decide which species we are going to eradicate for the benefit of another?”
Eradicating bullfrogs is much different from Grasso’s childhood, when he did bullfrog rescue operations, scooping them up from drying ponds with his ball cap and moving them to a nearby river.
“We have an invasive turtle, the red-eared slider, not in the park but adjacent, and I can’t even do that work,” he said.
Grasso said the reintroduction of native Western pond turtles in Yosemite has stopped for now. He said turtles involved in that new program were sent back to a San Francisco zoo facility while more research is conducted.
Other non-native animals and plants have been killed in Yosemite, including some species of fish in the High Sierra. But eradicating an invasive species is not always a goal.
The non-native crayfish, for example, is now a major food source for the native, recently-returned river otters in Yosemite, Grasso said. (The otters also eat red-legged frogs.) Ecological work can create unintended consequences, he said, and deciding what to do next takes a lot of forethought and research.
Returning home to the Sierra Nevada
Yosemite’s expertise eradicating bullfrogs is being shared with other regions through a new collaborative of researchers.
Grasso hopes the work being done will help restore red-leggeds in other areas of California where they’ve disappeared. The frogs’ historic range has been greatly diminished. The frogs are doing better on the Coast Ranges than in the Sierra.
“The significance is, if we can get the frogs established in Yosemite Valley,” Grasso said, “hopefully we can use this as a pinch point to start releasing in other known historic localities, not only in Mariposa County, but Tuolumne County, and in Fresno County.”
“Protecting vulnerable species like red-legged frogs maintains the park’s biodiversity as nature envisioned,” said Yosemite Conservancy President Frank Dean.
Visiting rangers from the Galápagos Islands in Ecuador released the first frogs in Cook’s Meadow on Friday as Yosemite Superintendent Michael Reynolds stood quietly behind a crowd of spectators, watching with pride.
“This is a major milestone,” Reynolds said, “in our work to reestablish a species that contributes to a healthy park ecosystem.”